There are known in the art two main processes of ink jet printing. In one process, usually called continuous ink-jet printing, a stream of ink drops are electrically charged and then are deflected by an electric field either directly or indirectly onto the substrate. Inks used in continuous ink-jet printing are electrically conductive and typically, are water based inks. The viscosity of such inks is typically 2 or 3 centipoise (cps) and generally does not exceed 10 cps.
In the second process, usually called Drop on Demand (DOD) ink-jet printing, the ink supply is regulated by an actuator such as a piezoelectric actuator. The pressure produced during the actuation forces a droplet through a nozzle or nozzles onto the substrate. Inks for DOD ink-jet printing do not need to be conductive and their viscosity is typically between 2 and 40 cps.
One type of prior art ink-jet ink is a water based ink-jet ink. A drawback of prior art water based ink-jet inks is that they dry slowly on paper. Another drawback of prior art water based inks is that they can be washed off the paper with water and they can be smudged with a wet finger.
Another type of prior art ink-jet ink is an ultra violet (UV) curable ink. Typically, prior art UV curable inks include polymerizable monomers or oligomers or a mixture thereof, all of which are miscible in organic solvents and are not miscible in water.
UV curable ink-jet inks based on organic solvents are particularly useful for printing on hard, non-absorbing substrates such as plastics. For example, U.S. Pat. No. 5,270,368 to Lent et al. describes a UV curable ink-jet ink for printing on printed circuit boards. Similarly, Published European Application No. 0,407,054 A1 describes an ink-jet ink for printing on plastic and metal surfaces where the ink is characterized by good adhesion properties.
One drawback of UV curable ink-jet inks based on organic solvents is their limited usefulness in printing on absorbing surfaces, such as paper. When printed on paper the inks penetrate through the paper and are seen from the back side of the paper. This is known in the art as "strike-through". Another drawback of such inks is that organic solvents, in particular volatile organic solvents, are potentially environmentally hazardous.